Abiotic Stress and Impact on Yield.
Water deficit is a common component of many plant stresses. Water deficit occurs in plant cells when the whole plant transpiration rate exceeds the water uptake. In addition to drought, other stresses, such as salinity and low temperature, produce cellular dehydration (McCue and Hanson, 1990).
Salt (and drought) stress signal transduction consists of ionic and osmotic homeostasis signaling pathways. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. The pathway regulating ion homeostasis in response to salt stress has been reviewed recently by Xiong and Zhu (2002a).
The osmotic component of salt-stress involves complex plant reactions that are possibly overlapping with drought- and/or cold-stress responses. Common aspects of drought-, cold- and salt-stress response have been reviewed by Xiong and Zhu (2002). These include:
Abscisic acid (ABA) biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.
Based on the commonality of many aspects of cold, drought, and salt stress responses, it can be concluded that genes that increase tolerance to cold or salt stress can also improve drought stress protection. In fact, this has already been demonstrated for transcription factors (in the case of AtCBF/DREB1) and for other genes such as OsCDPK7 (Saijo et al. (2000)), or AVP1 (a vacuolar pyrophosphatase-proton-pump, Gaxiola et al. (2001)).
Heat stress often accompanies conditions of low water availability. Heat itself is seen as an interacting stress and adds to the detrimental effects caused by water deficit conditions. Evaporative demand exhibits near exponential increases with increases in daytime temperatures and can result in high transpiration rates and low plant water potentials (Hall et al. (2000)). High-temperature damage to pollen almost always occurs in conjunction with drought stress, and rarely occurs under well-watered conditions. Thus, separating the effects of heat and drought stress on pollination is difficult. Combined stress can alter plant metabolism in novel ways; therefore, understanding the interaction between different stresses may be important for the development of strategies to enhance stress tolerance by genetic manipulation.
Plant Pathogens and Impact on Yield.
While a number of plant pathogens exist that may significantly impact yield or affect the quality of plant products, specific attention is being given in this application to a small subset of these microorganisms. These include:
Sclerotinia. 
Sclerotinia sclerotiorum is a necrotrophic ascomycete that causes destructive rots of numerous plants (Agrios (1997)). Sclerotinia stem rot is a significant pathogen of soybeans in the northern U.S. and Canada.
Botrytis. 
Botrytis causes blight or gray mold, a disease of plants that infects a wide array of herbaceous annual and perennial plants. Environmental conditions favorable to this pathogen can significantly impact ornamental plants, vegetables and fruit. Botrytis infections generally occur in spring and summer months following cool, wet weather, and may be particularly damaging when these conditions persist for several days.
Fusarium. 
Fusarium or vascular wilt may affect a variety of plant host species. Seedlings of developing plants may be infected with Fusarium, resulting in the grave condition known as “damping-off”. Fusarium species also cause root, stem, and corn rots of growing plants and pink or yellow molds of fruits during post-harvest storage. The latter affect ornamentals and vegetables, particularly root crops, tubers, and bulbs.
Drought-Disease Interactions.
Plant responses to biotic and abiotic stresses are governed by complex signal transduction networks. There appears to be significant interaction between these networks, both positive and negative. An understanding of the complexity of these interactions will be necessary to avoid unintended consequences when altering plant signal transduction pathways to engineer drought or disease resistance.
Transcription Factors (TFs) and Other Genes Involved in Both Abiotic and Biotic Stress Resistance.
Despite the evidence for negative cross-talk between drought and disease response pathways, a number of genes have been shown to function in both pathways, indicating possible convergence of the signal transduction pathways. There are numerous examples of genes that are inducible by multiple stresses. For instance, a global T×P (transcriptional profile) analysis revealed classes of transcription factor that are mainly induced by abiotic stresses or disease, but also a class of transcription factors induced both by abiotic stress and bacterial infection (Chen et al. (2002a)).
Implications for Crop Improvement.
Plant responses to drought and disease interact at a number of levels. Although dry conditions do not favor most pathogens, plant defenses may be weakened by metabolic stress or hormonal cross-talk, increasing vulnerability to pathogens that can infect under drought conditions. However, there is also evidence for convergence of abiotic and biotic stress response pathways, based on genes that confer tolerance to multiple stresses. Given our incomplete understanding of these signaling interactions, plants with positive alterations in one stress response should be examined carefully for possible alterations in other stress responses.